Poly(ADP-ribose) synthetase is a chromatin bound enzyme thought to be involved in some types of DNA repair. In intact cells the enzyme is inactivated by a non-lethal heat shock in both Drosophilia and Hela cells. The repair of single strand DNA chains breaks was efficiently conducted in both cell types following heat shock indicating that this type of repair did not require synthetase action. Repair of this type of lesion was also not affected by synthetase inhibitors such as 3-aminobenzamide or 5-methylnicotinamide. The clonal growth of CHO cells was dramatically reduced by brief exposure to 3-acetylaminobenzamide, one of the most potent synthetase inhibitors known. 3-aminobenzamide also caused the death and resorption of mouse fetuses. Thus, the synthetase probably plays some roles in cells other than, or in addition to, DNA repair. This synthetase inhibitor had no effect on tyrosine transaminase induction in HTC cells nor on the methylation of deoxycytidine in DNA. Thus, there is no evidence for a role of the synthetase in gene expression in these cells.